1. Field of the Invention
This invention relates generally to measurement and data acquisition systems and, more particularly, to instrumentation amplifier design.
2. Description of the Related Art
Scientists and engineers often use measurement systems to perform a variety of functions, including measurement of a physical phenomena or unit under test (UUT), test and analysis of physical phenomena, process monitoring and control, control of mechanical or electrical machinery, data logging, laboratory research, and analytical chemistry, to name a few examples.
A typical measurement system comprises a computer system with a measurement device or measurement hardware. The measurement device may be a computer-based instrument, a data acquisition device or board, a programmable logic device (PLD), an actuator, or other type of device for acquiring or generating data. The measurement device may be a card or board plugged into one of the I/O slots of the computer system, or a card or board plugged into a chassis, or an external device. For example, in a common measurement system configuration, the measurement hardware is coupled to the computer system through a PCI bus, PXI (PCI extensions for Instrumentation) bus, a GPIB (General-Purpose Interface Bus), a VXI (VME extensions for Instrumentation) bus, a serial port, parallel port, or Ethernet port of the computer system. Optionally, the measurement system includes signal conditioning devices which receive field signals and condition the signals to be acquired.
A measurement system may typically include transducers, sensors, or other detecting means for providing “field” electrical signals representing a process, physical phenomena, equipment being monitored or measured, etc. The field signals are provided to the measurement hardware. In addition, a measurement system may also typically include actuators for generating output signals for stimulating a UUT.
Measurement systems, which may also be generally referred to as data acquisition systems, may include the process of converting a physical phenomenon (such as temperature or pressure) into an electrical signal and measuring the signal in order to extract information. PC-based measurement and data acquisition (DAQ) systems and plug-in boards are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor, among others.
Typically, in a measurement or data acquisition process, analog signals are received by a digitizer, which may reside in a DAQ device or instrumentation device. The analog signals may be received from a sensor, converted to digital data (possibly after being conditioned) by an Analog-to-Digital Converter (ADC), and transmitted to a computer system for storage and/or analysis. Then, the computer system may generate digital signals that are provided to one or more digital to analog converters (DACs) in the DAQ device. The DACs may convert the digital signal to an output analog signal that is used, e.g., to stimulate a UUT.
Multifunction DAQ devices typically include digital I/O capabilities in addition to the analog capabilities described above. Digital I/O applications may include monitoring and control applications, video testing, chip verification, and pattern recognition, among others. DAQ devices may include one or more general-purpose, bidirectional digital I/O lines to transmit and received digital signals to implement one or more digital I/O applications.
Generally, signals that are being measured using a DAQ system are first routed from a particular channel via a multiplexer. The signals then enter an instrumentation amplifier, typically a programmable gain instrumentation amplifier (PGIA). The PGIA typically applies a specified amount of gain to an input signal, which raises the signal to a higher level and ensures proper A/D conversion. The amplifier may also convert differential input signals applied to the DAQ board to a single-ended output so that the ADC can correctly digitize the data. The ADC may then sample and hold the signal until the signal is digitized and placed into a FIFO buffer on the board. In the FIFO, the digitized signal is ready to be transferred from the board to computer memory via the PC bus for further processing. PGIA performance is generally considered an important aspect of DAQ systems. For example, the PGIA must settle before the A/D conversion takes place or the converted data may be inaccurate. The time needed to amplify the signal to the higher level while maintaining the accuracy of the ADC—in other words, the settling time of the instrumentation amplifier—may also be a concern when using plug-in DAQ boards.
Traditional PGIAs typically use operational amplifiers (op-amps) with multiplexers to switch feedback networks. The gain multiplexers configured as part of the PGIA circuitry are generally considered primary performance limiters due to multiplexer capacitance. Eliminating the capacitive load the multiplexers place on the PGIA circuit can lead to improved performance and potential elimination of large signal distortion, as well as substantial decrease of noise and frequency response aberrations.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.